It is known to manufacture hollow metallic turbomachine blades, in particular fan blades for a jet engine, by superplastic forming and diffusion bonding metal workpieces, the workpieces forming pressure and suction surfaces of the blade. These metal workpieces may include elementary metal, metal alloys and metal matrix composites. At least one of the metal workpieces may be capable of superplastic extensions. In one known process the surfaces of the workpieces to be joined are cleaned, and at least one surface of one or more of the workpieces is coated in preselected areas with a stop-off material to prevent diffusion bonding. The workpieces are arranged in a stack and the edges of the workpieces are welded together, except where a pipe is welded to the workpieces, to form an assembly. The pipe enables a vacuum, or inert gas pressure, to be applied to the interior of the assembly. The assembly is placed in an autoclave and heated so as to “bake out” the binder from the material to prevent diffusion bonding. The assembly may then be evacuated, using the pipe, and the pipe is sealed. The sealed assembly is placed in a pressure vessel and is heated and pressed to diffusion bond the workpieces together to form an integral structure. Diffusion bonding occurs when two matt surfaces are pressed together under temperature, time and pressure conditions that allow atom interchange across the interface. The first pipe is removed and a second pipe is fitted to the diffusion bonded assembly at the position where the first pipe was located. The integral structure is located between appropriately shaped dies and is placed within an autoclave. The integral structure and dies are heated and pressurised fluid is supplied through the second pipe into the interior of the integral structure to cause at least one of the workpieces to be superplastically formed to produce an article matching the shape of the dies.
In addition to the hollow structure just described, it is also known to insert a membrane 2 between the metal workpieces (i.e. first and second layers 4, 6 which form pressure and suction surfaces 18, 16 respectively) prior to the above described process (see FIG. 1 for example). The location of diffusion bonds between the membrane and the adjacent workpieces can be controlled by applying the stop-off material to preselected areas on each side of the membrane (or respective workpieces). When the blade is subsequently expanded, the membrane adheres to the workpieces where the diffusion bond is allowed to form and thereby provides an internal structure.
Various internal structures have been proposed and different preselected patterns of the stop-off material are required to achieve these structures. For example, U.S. Pat. No. 5,479,705 discloses an internal structure with a Warren girder type cross-section, which is formed by a pattern of alternating strips 10, 12 on either side of the membrane where there is no stop-off material (see FIG. 2 for example). Such blades are better at withstanding a bird strike due to the presence of a crumple zone which allows them to yield rather than fracture. However, during the expansion of the blade the membrane may stick to the pressure surface workpiece in the region between the blade tip and the edge of the aforementioned strips, since the stop-off material adheres to the second layer 6. This sticking may inhibit the expansion during the superplastic process. This can be particularly problematic for the pressure surface since it should move the most during the expansion process and since the shape of the pressure surface of a fan blade is aerodynamically very important.
To prevent the membrane from sticking to the second layer 6, a pattern comprising the previously mentioned strips 10, 12 but with additional dots 14 on the suction side has been proposed, as shown in FIG. 2. The strips 10 are on the suction side and the strips 12 are on the pressure side (see FIG. 2). The ends of the strips 10 on the suction side are spaced further from the blade tip than the ends of the strips 12 on the pressure side. With such a bond pattern, the dots 14 are in the tip area of the blade and are in line with the strips 10 on the suction side 16 of the membrane.
The dots 14 define further regions in which there is no stop-off material and in which a diffusion bond is allowed to form. The dots 14 therefore ensure that the membrane 2 adheres to the first layer 4 at the tip region and keep the membrane 2 away from the second layer 6 so as not to inhibit the expansion of the pressure surface 18 (see FIG. 4).
The membrane 2 is further provided with a D-shaped recess 20 which extends into the membrane 2 from the blade tip towards the strips 10, 12, FIG. 3. During the superplastic forming process, the pressurised fluid is supplied through the pipe 22 into the interior of the blade via the D-shaped recess 20. The D-shaped recess 20 is intended to allow the fluid to enter the blade on either side of the membrane 2 and thus should provide an equal pressure difference on both sides of the membrane 2 to form the internal structure within the blade.
As a result of the bonds between the membrane 2 and the first and second layers 4, 6 formed by the strips 10, 12 and dots 14, the membrane 2 is deformed along a series of axes, as indicated by lines 24 in FIG. 3. The axes are drawn between the origin of the D-shaped recess 20, the bonds at the dots 14 on the suction side 16 and the bonds along the strips 12 on the pressure side. Furthermore, the membrane 2 is deformed between the strips 10 on the suction side and the strips 12 on the pressure side.
Conventionally, the D-shaped recess 20 has a radius of 6 mm and the dots 14 on the suction side 16 are located at a fixed distance from the edge of the membrane, the distance being typically between two and three times (inclusive) the radius of the D-shaped recess.
However, this known design does not reliably achieve an equal pressure on both sides of the membrane 2 during the superplastic forming process. Consequently, the required internal structure may not be correctly formed by the membrane 2, and may result in ‘Arched Webs’. In order to distribute the pressurised fluid from the D-shaped recess 20 across the entire blade, the region between the tip of the blade and the edges of the slots 10, 12 must expand to form a manifold 26. However, if the pressurised fluid does not attain an equal pressure on either side of the membrane 2 during initial inflation, the membrane 2 is forced into the lower pressure side resulting in arched webs. Such components are unacceptable and therefore result in waste and consequently increased component cost.